This invention relates to a torch ignitor of the type that is utilized in a combustion device to initiate the combustion of a combustible mixture within the combustion device. More specifically, the invention relates to a new and unique torch ignitor which, by means of an electric spark, ignites a small combustible mixture flowing through the torch ignitor to create a flame that is emitted at the front end of the torch ignitor and is directed into a main combustion chamber to ignite a much larger combustible mixture that is introduced into the combustion chamber.
The use of an electric spark type torch ignitor to ignite a combustible mixture in a combustion device is well known. Various examples of this type of torch ignitor are found in prior patents. A preliminary novelty search conducted in respect of this invention developed the following U.S. patents.
______________________________________ Benson 2,493,743 Raskin 2,880,792 Frank 2,285,704 Johnston 3,330,985 Piros et al 2,668,592 Morishita 4,215,979 ______________________________________
The Raskin U.S. Pat. No. 2,880,792 is relevant to the present invention. It shows an ignitor in which the electrode tip is located in a sheltered area of the ignitor so that it is not exposed to the main combustion chamber of the combustion device. Sheltering of the electrode tip in a location outside the main combustion chamber is desirable in order to minimize tip erosion.
The Raskin device comprises a first tube onto the front end of which the rear end of a second tube is partially telescoped. Gas is introduced at the rear end of the first tube and passes through the first tube. Air admission slots are provided in the wall of the second tube just beyond where it partially telescopes onto the first tube. Air enters these slots to mix with the gas that has passed through the first tube. The mixture continues through the second tube passing over the ignitor electrode tip which is located within the second tube toward the front end thereof. When the ignitor is energized, a spark is created between the electrode tip and the wall of the second tube, and this spark ignites the combustible mixture. The combustible mixture is discharged as a flame at a nozzle which is formed at the front end of the second tube.
The ignitor structure of the Raskin device comprises a spark plug that is arranged coaxially with the first tube and is screwed into a threaded opening in a spark plug holder structure that is itself threaded onto the rear end of the first tube in the vicinity of where the gas is introduced into the first tube. The ignitor comprises a metal rod that passes coaxially through both tubes, terminating within the second tube in the electrode tip. The tip is bent at a right angle to the main extent of the wire rod. A threaded connector element threads into one end of the spark plug and into the rear end of the metal rod to connect the two. A porcelain insulating sleeve is disposed around the concentric portion of the metal rod, and means are provided on the wall of the first tube for centering the insulating sleeve, and hence the main extent of the metal rod, with respect to the two tubes.
The remaining patents mentioned above show other arrangements for constructing electric type torch ignitors and/or for sheltering the electrode tip.
Virtually all these prior devices appear to have a fixed mounting on the combustion device, typically by some type of a threaded joint located at or near the rear end of the device. Hence axial expansion and contraction, that occur in response to changing thermal conditions, could create a certain amount of change in the location of the tip relative to its surrounding structure depending upon the particular temperature changes to which the particular device is exposed. If an attempt were made to maintain a fixed location for the electrode tip and provision were not made for compensating for axial expansion and contraction, there could be instances where excessively high stresses are induced in an ignitor.
Where an ignitor comprises an electrode structure which is embedded in or otherwise surrounded by an insulating ceramic material, failure to compensate for thermally induced mechanical stresses, while attempting to maintain tip location, may result in ignitor failure due to fracture of the ceramic material. It does not appear that the prior art has addressed this potential problem.
The present invention relates to a new and improved torch ignitor in which the location of the electrode tip is accurately maintained even though the device itself may be subjected to a range of thermal conditions. The torch ignitor of the present invention also comprises a construction in which thermally induced mechanical stresses are minimized. As a result, the torch ignitor of the present invention is seen to be capable of providing a longer service life with less likelihood of breakage or malfunction. Moreover, if it becomes necessary to replace the ignitor, such replacement can be conveniently and expeditiously accomplished once access to the rear of the torch ignitor has been attained.
Briefly, the torch ignitor of the present invention comprises an ignitor housing tube having its front end fixedly referenced to a shelter chamber for the electrode tip. The electrode tip of the ignitor is fixedly referenced to the shelter chamber by inserting the ignitor into the ignitor housing tube until a shoulder on the ignitor comes into contact with an abutment on the wall of the ignitor housing tube that is in a predetermined relationship to the means which locates the ignitor housing tube with respect to the shelter chamber. In this way the electrode tip is located in a desired position within the shelter chamber when the ignitor is inserted into the ignitor housing tube.
Maintenance of the axial position of the ignitor within the ignitor housing tube is achieved by means of several parts which serve to resiliently urge the ignitor axially within the ignitor housing tube to maintain the ignitor shoulder in contact with the abutment. These several parts comprise a retainer cap that is shaped to fit onto the open rear end of the ignitor housing tube and to engage the ignitor through an O-ring seal that seals between the outside of the ignitor and the inside of the ignitor housing tube at the rear of the torch ignitor. A retaining clip spring is fastened between a fixed location on the combustion device and the retainer cap and serves to urge the retainer cap in the forward direction so that the ignitor is in turn maintained in contact with the abutment the ignitor housing tube's wall. The retaining clip spring has a certain resiliency which allows for thermally induced expansion and contraction of the assembled parts whereby expansion is taken up by increased flexing of the retaining clip spring and contraction by decreased flexing. In this way, any build-up of major mechanical stresses in the ignitor housing tube and in the ignitor are avoided.
The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a presently preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.